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Need help diagnosing engine problem

walkman

walkman

Well Known Member
Help, I'm stumped!

Background. 2004 RV-8. I purchased in early Feb this year with approx 800 SNEW. Superior XP-360 with Bendix RSA-5 injection system. Engine now has approx 1,000 hours. No purge valve. Engine originally had 2 Slick mags, about 300 hours each, with Tempest massive electrode plugs. I replaced one mag with an EFII EI system, I have also replaced the Holley fuel injection pump with an EFII boot pump module. Compressions at pre-purchase all high 70's. No significant issues with oil analysis performed every change since new including one I just did a month or so ago. I'm running 100LL.

Problem. When engine is hot, I get a VERY lopey, rough idle. It will idle for a second or two at 1,000 or wherever I set it, then the idle will drop out down 800 or lower for a second, then back up to 1,000. It feels like a cylinder is dropping out for a second then coming back online, but I see no indication in the EGT/CHT to identify if this is in fact the case or identify which cylinder. Problem seems to be worse the hotter the engine is. When its hot I get a lot of sputtering, occasional backfire, and sometimes feels like its almost about to quit.

The airplane did not do this when I picked it up, it has gradually gotten worse as the year has progressed (and the WX has gotten warmer?). Problem did NOT suddenly start with either the pump or the EI upgrades.

The engine does make good power and performs well in the air.

What I have done:
-2 new sets of automotive plugs - no difference
-cleaned gaped ohmed out all aviation plugs (900-1k ohms) no difference
-mag is dead nuts on 25 BTDC
-all ignition leads are in ex cond.
-problem does not change when running on mag, EI, or both anyway
-problem does not change when fuel pump on or off (pump works properly)
-problem doesn't change with different mixtures on the ground
-for a few seconds after a cold start runs rough them smooths out, but.....
-checked all exhaust valves and all valve springs, no broken springs, all valves smooth as silk, no sticking, and just the tiniest bit of wobble (no dial gauge but seems fine)
-all new intake gaskets and hoses/clamps - no change

I'm stumped. I'm going to try the "clear tube" test and see if I'm somehow getting air in the fuel lines, clean all my fuel filters and strainers, and double check all the adjustments on the fuel servo this weekend.

Any guesses?

Here's a short video showing the behavior. I'm not touching the throttle.
 
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The Backfire sounds like electrical.May I ask, key or toggle switches?Any common grounds?I wouldn't rule anything out with the fuel system yet,just saying.
RHill
 
rhill said:
The Backfire sounds like electrical.May I ask, key or toggle switches?Any common grounds?I wouldn't rule anything out with the fuel system yet,just saying.
RHill
Click to expand...

Originally ACS key switch when it was dual mags. Now toggle switches for master/alt (DPST), EI (potter switch breaker), Mag/start (DPDT On-off-(on))
 
Ignition shutting off intermittently may casue afterfiring, but I doubt it will cause backfiring, they are two very different occurences with different causes. I would think if it were ignition related you would see other symtoms besides a erratic idle. I would start looking for induction leaks and idle mixture setting.
 
Mike H said:
Ignition shutting off intermittently may casue afterfiring, but I doubt it will cause backfiring, they are two very different occurences with different causes. I would think if it were ignition related you would see other symtoms besides a erratic idle. I would start looking for induction leaks and idle mixture setting.
Click to expand...

+1

I would go no further until I was sure the idle mixture was set properly. Induction leaks are included here, as they impact the mixture. You can also have someone watch the exhaust as you drop to idle. Any puffs of black smoke would indicate you are going rich, but I am guessing that you are going lean. A partially plugged injector could also cause one cylinder to go lean, but you would probably notice differences in the EGTs.

Larry
 
If it doesn't happen cold you are getting vapor lock. 100LL will also vapor lock especially if the lines aren't insulated and kept away from radiant exhaust heat. A gascolator FWF will make it worse particularly Van's POS blue gascolator since it does not have a standpipe.
 
Mike H said:
Ignition shutting off intermittently may casue afterfiring, but I doubt it will cause backfiring, they are two very different occurences with different causes. I would think if it were ignition related you would see other symtoms besides a erratic idle. I would start looking for induction leaks and idle mixture setting.
Click to expand...

Backfiring isn't usual, its happened once or twice. Pretty sure I've eliminated ignition as a source anyway, as the problem manifests itself equally when running either or both ignition systems.

I've replaced all intake gaskets, hoses, and clamps with new. Not sure where else there could be a leak of unmetered air.
 
lr172 said:
+1

I would go no further until I was sure the idle mixture was set properly. Induction leaks are included here, as they impact the mixture. You can also have someone watch the exhaust as you drop to idle. Any puffs of black smoke would indicate you are going rich, but I am guessing that you are going lean. A partially plugged injector could also cause one cylinder to go lean, but you would probably notice differences in the EGTs.

Larry
Click to expand...

I've looked for a plugged injector by slowly leaning towards ICO and watching EGTs. I don't see one rising or falling any more than the others.

Nevertheless this weekend I will drop all four into graduated baby bottles and run the pump to see if one is significantly different that the others.
 
rocketbob said:
If it doesn't happen cold you are getting vapor lock. 100LL will also vapor lock especially if the lines aren't insulated and kept away from radiant exhaust heat. A gascolator FWF will make it worse particularly Van's POS blue gascolator since it does not have a standpipe.
Click to expand...

I have thought of that, and honestly that's the direction I'm leaning (groan what a punny guy). However, all the fuel lines FWF are fire sleeved, which I guess insulates them, except from distributor to injectors.

I really didn't expect 100LL to act like this though.
 
walkman said:
I have thought of that, and honestly that's the direction I'm leaning (groan what a punny guy). However, all the fuel lines FWF are fire sleeved, which I guess insulates them, except from distributor to injectors.

I really didn't expect 100LL to act like this though.
Click to expand...

You could reduce to idle at altitude and see if the problem remains. You shouldn't be getting vapor lock at altitude / cruise.
 
walkman said:
I've looked for a plugged injector by slowly leaning towards ICO and watching EGTs. I don't see one rising or falling any more than the others.

Nevertheless this weekend I will drop all four into graduated baby bottles and run the pump to see if one is significantly different that the others.
Click to expand...

Have you checked the mixture? I would do this before the injector test.
 
I had a similar problem with an IO 320. It ran fine except for idle when the engine was anything other than cold. If the engine was cold it idled smooth as could be.

Ended up finding that I was developing vapor in the line between the injector and the flow divider. I determined this with a clear line into the injector (no bubbles) and clear line out (bubbles when the engine was warm/hot). Went through all sorts of effort to isolate, insulate and cool the fuel pump and lines up to the injector and no joy. Had the flow divider pressure setting set to factory specs (increased about 1PSI I think) and stopped the problem. The increased pressure in the line stopped the vapor. Engine still runs fine- now 10 yrs later.
 
Fine cold, bad hot? Then you have the same issue I've just gone through and the problem is the fuel heating up on it's way to the servo, and perhaps being agitated along the fuel path such that cavitation occurs and/or you're sucking air to create entrained vapor. Then it makes air bubbles because of the 10:1 pressure drop passing though the servo (like opening a beer can). Put a three-inch length of clear tubing in-line just before the divider when the engine's hot and acting up. You can see the bubbles.

The first RSA I installed was quick to gag and sputter at lower power settings, even during descent, after minimal heating. Prop could stop during rollout! The next one is fine unless I idle from cold for more than 15 minutes (like departing from OSH), or more than five minutes after landing. So not all RSA's are alike. Airflow Performance says they can set their calendar by the number of calls for this sort of problem as the hotter weather comes on.

Check everything else you want, but even running ridiculously rich at idle does not cure the symptom.

John Siebold
 
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There is a SL on the spider that allows for higher pressures which reduces the idle vapor problem. Don at Airflow Perfomance did mine and can probably help you.
 
My AFP system has idled crummy when OAT's above about 85-90 when taxiing in after a flight. It has done this for 13 years. The metered fuel pressure is so low at idle, that boiling (intermittent) occurs in the lines downstream of the servo. I'm not sure how the RSA systems work, but one can put smaller nozzles in each of the injectors on the AFP system, PROVIDED one can still achieve full fuel flow at takeoff. These serve to increase the pressure in the metered section at idle (and other regimes also). I've not heard of anything done within the flow distributor to do this, but it certainly is possible.

So, vapor lock is likely not the cause - percolation downstream of the servo might be a better description.
 
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